[vol. cvii, part 4] w. D. GILL ON THE SIWALIK SERIES 375 14 THE STRATIGRAPHY OF THE SIWALIK SERIES IN THE NORTHERN POTWAR, PUNJAB, PAKISTAN BY WILLIAM DANIEL GILL, D.SC. F.G.S. Read 20 June, 1951 [PLATES XX-XXII] CONTENTS Page I. Introduction .................................................................. 376 II. The Jhamat area ............................................................ 378 III. The eastern part of the Soan syncline, near Rawalpindi ......... 380 (a) Introduction and previous work ................................. 380 (b) Structure ............................................................... 381 (c) Stratigraphy ......................................................... 381 IV. General summary and conclusions ....................................... 388 (a) Siwalik facies and thickness variation in the northern Potwar ............................................................... 388 (b) The relationship of the Siwalik phase of Himalayan ore- genesis to the Pliocene/Pleistocene boundary in North- West India ......................................................... 390 V. List of references ............................................................ 394 SUMMARY Detailed mapping of the Jhamat area in the west and of the Soan syncline in the east forms the basis of revision of the stratigraphy of the Siwalik Series in the northern Potwar region. The succession is here affected by facies changes of regional significance :-- 1. Near the Indus River the uppermost 3000 feet of the Middle Siwalik sequence (Upper Nagri and Dhok Pathan stages) contain thick beds of con- glomerate which die out eastwards and south-eastwards into sandstones and clays. Farther eastwards, across the Soan syncline, a clay facies develops at progressively lower horizons in the Nagri Stage, replacing a considerable portion of the massive sandstones of the type area. The facies change is accompanied by a reduction in thickness. The facies variation in the Middle Siwalik rocks is indicative of a channel environment in the west at Jhamat, clearly coinciding with the line of the present Indus River and grading eastwards into deposits of " flood-plain " type. 2. The Lower Siwalik rocks show a development of coarser facies in the opposite direction to that noted in the Middle Siwalik. At the eastern end of the Soan syncline the lowest 1500 feet of the Chinji Stage, which in the west is almost entirely of clay-shale facies, contain thick beds of hard sandstone and the sequence is very similar to that of the underlying Kamlial Stage. In the Soan area, the complete Siwalik succession from the Kamlial Stage to the Pinjor Stage (Villafranchian) is free from any marked unconformities but is overlain with strong unconformity by post-Siwalik Pleistocene beds--the Lei Conglomerates. This interpretation differs greatly from that of de Terra (1936, 1939), who placed the first great unconformity of the area at the base of the Tatrot Stage. The revision explains Pilgrim's reluctance to include the Villafranchian in the Pleistocene, since in the Indian Siwalik belt this stage forms the highest member of a conformable succession which was intensely folded and subjected to peneplanation before the next deposition took place. The decision at the International Congress in 1948 to adopt the Villafranchian as the base of the Pleistocene places the Siwalik phase of Himalayan orogenesis within the Pleistocene System. 376 w.D. GILL O1~1 TItE SlWALIK S~I~I~.S [VO1. cvii, I. II~ITRODUCTI051 Physiographically, the Potwar region may be defined as the elevated, dissected peneplain which lies between the Indus and Jhelum rivers, forming a platform between the Salt Range in the south and the outer ranges of the sub-Himalaya, the Kala Chitta and Margala Hills in the north (see Fig. 1). ~,,o~, }" 'F . I~R.... ,~,~ , ,Jh_~o.m~L..--p.0..~ /" . ".' . 9 __ L.:<'~.d,/~_~ ..~,,.h ...... ~ ..,../--'" .- -.......... 9..-............../...~ . .~ ........ ..............~,..../:< ." . "./." /'.' - // .-..'.'." ." . "." To,oo.,~'......, ........ 9/:ChOokWOy 9 . / 9./.. ............. "/i 9 " "/~ .... 9"'" ' " ............. ".---: 9.. 9 ~. 9._,-or. :~/J. ".../ ~i~!l~~:.:.:..:.: :.~y :. ....:.. 9 ,?o~: :.. 72~ 73~ Major told axis in Stwohk br ~ E0cenr and [~ Murrcr ~ Slwahk .... Antlchnr ~ Synchnr Pre- Eocene o__ ,o zo .~o H~I~ .Sc~i~ _ _, , FIo. 1.--Geological sketch-map of the Potwar region, Punjab, Pakistan. The Potwar forms a part of the folded geosynclinal accumulation of Middle and Upper Tertiary freshwater deposits which border the Himalayan mountains throughout their length from Assam to the North-West Frontier. Anderson (1927) has conveniently grouped these deposits in the " Nimadric System ", a name which, being a compound of " Himadri ", meaning snowy range and an alternative for Himalaya, with the Sanskrit prefix " ni ", meaning under, nether or downward, indicates their geological position. The general succession, classification and average thicknesses of the Nimadric rocks in the north-west portion of the Himalayan foredeep can be summarized as follows, the Siwalik Series being given in detail :-- part 4] rs THE NORTHERN POTWAR 377 TABLE I.--GENERAL SUCCESSION ThiCKNESS IN SERIES STAGE AND LITHOLOGICAL CHARACTERS FE~T (APPROX.) |BouLDER CONGLOMERATE STAGE. Coarse sand, | gravel and conglomerate. | 5) ]PI~JOR STAGE. Pink silt, sandstone, gravel,| o i~ conglomerate (Soan syncline). 4500-6000 ]Pebbly sandstones of Rhotas, Hatar (Salt| | Range) and Kangra. | (TATROT STAGE. Clay, sandstone, conglomerate.J [ Bhander Beds (Salt Range). 500 DHOK PATHAN STAGE. Variegated shales, sand- 1000 ~ stones. Conglomerate (Jhamat). NAoRI STAGE. Pepper-coloured sandstones, 1500 (Salt Range)- drab and red shales. 6000 |UPPER CHINJI STAGE. Brick-red shales, grey~ | sandstone. 1500 (Salt Range)- | LOWER C~INJI STAGE. Bright-red shales (Pot- | 6000 war). Nahan Sandstone of Kangra. J i| KAMr~IAL STAGE. Hard red and brown ridge- 500-2500 ] forming sandstones, deep red shales (Potwar). Lower Nahan Sandstones of Kangra. MURREE Up to 8000-9000 Maximum total 31,000 In general, the lower part of the Nimadric System, up to the base of the Middle Siwalik, is composed of fine- or medium-grained sandstone in beds ranging from a few inches to hundreds of feet in thickness, alternating with beds of brightly coloured clay, clay-shale and siltstone. The main lithe- logical types and the sedimentary characteristics of these lower beds of the system strongly suggest dominantly lacustrine rather than fluviatile con- ditions of accumulation. The Middle Siwalik sees the introduction of coarse sand-rock into the system, with evidence of fluviatile deposition. During Upper Siwalik times great thicknesses of coarse conglomerate, alternating with finer grades of sediment, were deposited. Similar coarse sediments occur locally at lower (Middle Siwalik) stratigraphical horizons towards the northern limit of the outcrops. A predominance of clay or of sandstone over extensive vertical ranges formed the original basis of subdivision of the Siwalik Series into stages ; these stages were subsequently adopted as zones on the evidence of the prolific vertebrate faunas, which have attracted world-wide attention. It must be emphasized, however, that the palaeontological zones lack sharp definition and that the beds are not throughout fossiliferous, either in the stratigraphical or the geographical sense. In these circumstances, lithology must often be relied upon in identifying the various subdivisions, and in nearly all cases forms the basis on which the zonal boundaries can be mapped. In view of the lateral variation of facies which is to be ex- pected in deposits of this type, it is not surprising that previous workers, particularly those engaged in mapping on a regional scale, have expressed difficulty in finding clear lines of demarcation between certain zones. Both 378 w.D. GILL ON THE SIWALIK SERIES [VO1. evil, Anderson (1927) and Cotter (1933) were critical of the Siwalik/Murree boundary as defined by Pilgrim and Pinfold (i.e. at the base of the Kam- lial Stage), and Cotter suggested raising the boundary between the series to the base of the Chinji Stage. The present paper demonstrates that the same difficulty applies to the boundaries of all the Siwalik stages of the northern Potwar. Here the entire succession is subject to facies changes on a regional scale, which yield important information on the problem of Siwalik palaeogeography. These facies changes are described in detail in two areas--the Jhamat area in the west and the Soan syncline in the east. Both areas have yielded other important information on the history of the Nimadric geosyncline in its later stages and, particularly the Soan area, on the date of its deformation. The field investigations on which the present paper is based were carried out whilst the author was engaged in the search for oilfields in the service of the Attock Oil Company, and it is by kind permission of the Directors of the Company that these results are published. II. THE JHAMAT AREA The map (P1. XX) shoves the massive Nagri sandstones at the western end of the Dhulian anticline in the south-east to be overlain by thin alternations of clay and sandstone of the Dhok Pathan Stage ; on the other hand, in the north-west, flanking the Nagri sandstones in the core of the Jhamat anticline, is a succession dominated by thick beds of conglomerate. The area is discussed in some detail by Cotter (1933, pp. 117-20), who regards the entire thickness of conglomeratic facies, amounting to about